Surface charge modified nanosurfactants for reduced retention by reservoir rock

ABSTRACT

Disclosed here compositions and methods suitable for injection of a nanosurfactant composition into a hydrocarbon-bearing formation for enhanced recovery operations. The nanosurfactant composition includes nanoassemblies that contain a petroleum sulfonate surfactant, mineral oil, a zwitterionic co-surfactant, and a cationic co-surfactant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patent application Ser. No. 15/917,097, filed on Mar. 9, 2018, which claims the benefit of and priority to U.S. Provisional Application Ser. No. 62/469,414, filed on Mar. 9, 2017; all of the above-referenced applications are incorporated by reference in their entireties into this application.

FIELD

This disclosure relates to nanoparticle compositions that are stable under high salinity and high temperature conditions. This disclosure is also directed to methods of producing these nanoparticle compositions and their use for improved and enhanced oil recovery applications.

BACKGROUND

Current waterflood operations result in about an average of 50% recovery of original oil in place. In conventional surfactant-based enhanced oil recovery (EOR) applications, tremendous amounts of surfactants are used to mobilize or solubilize oil. This is primarily due to a significant portion of the surfactants injected being adsorbed onto the rock near the wellbore area or diffused into the water-filled small pores. In order to deliver a specific concentration of surfactants to a distant oil location, large concentrations of surfactants need to be injected to account for the surfactants lost by adsorption or diffusion into dead-end pores before reaching the target oil. One way to mitigate the loss of surfactants is to increase the amount of surfactants in water, either by increasing the concentration of a given slug size of surfactants or increasing the slug size of a given concentration of surfactants, to deliver enough surfactants to the oil/water interface. Other approaches use sacrificial chemicals to passivate the rock surface and minimize surfactant adsorption. Overall, the approaches fail to maintain a constant concentration of surfactant over an extended period in the formation. These techniques also increase the cost of current surfactant-enhanced oil recovery techniques.

SUMMARY

Embodiments disclosed and described here address the shortcomings in the art such as lack of long term stability of EOR compositions under conditions of high salinity and high temperature. Embodiments include a nanosurfactant-containing composition suitable for injection into a hydrocarbon-bearing formation for enhanced recovery operations. In an embodiment, the EOR composition includes nanoparticles containing a sulfonate surfactant, a zwitterionic co-surfactant, and mineral oil. These nanosurfactants are delivered as aqueous compositions containing cations, such as sodium, calcium, magnesium, and potassium. The sulfonate surfactant and the zwitterionic co-surfactant form nanoparticles having a particle diameter ranging from about 10 nanometers (nm) to 100 nm. The nanoparticles are stable under high salinity and high temperature conditions. In certain embodiments, a substantial portion of the nanoparticles are stable for at least three months at temperatures of at least 100 degrees Celsius (° C.). The sulfonate surfactant can be selected from the group consisting of an alkyl sulfonate, an alkyl aryl sulfonate, and combinations thereof. In certain embodiments, the sulfonate surfactant is petroleum sulfonate. The zwitterionic co-surfactant can contain cocamidopropyl hydroxysultaine.

Embodiments disclosed and described here include methods for recovering hydrocarbons from a hydrocarbon-bearing formation. One such method includes introducing into the hydrocarbon-bearing formation a fluid containing nanoparticles. The nanoparticles are formed by mixing effective amounts of a sulfonate surfactant, a zwitterionic co-surfactant, and mineral oil. The method further includes driving the fluid through the hydrocarbon-bearing formation to displace hydrocarbons from the hydrocarbon-bearing formation; and recovering the displaced hydrocarbons. The sulfonate surfactant and the zwitterionic co-surfactant can form nanoparticles having a particle diameter ranging from about 10 nm to 100 nm. The nanoparticles are stable under high salinity and high temperature conditions. In certain embodiments, a substantial portion of the nanoparticles are stable for at least three months at temperatures of at least 100° C. The sulfonate surfactant can be selected from the group consisting of an alkyl sulfonate, an alkyl aryl sulfonate, and combinations thereof. The sulfonate surfactant can contain petroleum sulfonate. The zwitterionic co-surfactant can contain cocamidopropyl hydroxysultaine.

Embodiments disclosed and described here include methods for producing nanosurfactant-containing fluids suitable for injection into a hydrocarbon-bearing formation for enhanced recovery operations. One such method includes the steps of mixing a first aqueous mixture containing a sulfonate surfactant and a second aqueous mixture containing a zwitterionic co-surfactant in a reactor to form a third aqueous mixture. In the next step, a fourth aqueous mixture containing cations is mixed with the third aqueous mixture in the reactor to produce a fluid containing nanosurfactants with a particle diameter ranging from about 10 nm to 100 nm and containing the sulfonate surfactant and the zwitterionic co-surfactant. In certain embodiments, the first aqueous mixture containing the sulfonate surfactant further contains mineral oil and fresh water. The sulfonate surfactant can be selected from the group consisting of an alkyl sulfonate, an alkyl aryl sulfonate, and combinations thereof. The first aqueous mixture containing the sulfonate surfactant can further contain petroleum sulfonate, mineral oil, and fresh water. In certain embodiments, the second aqueous mixture is produced by mixing the zwitterionic co-surfactant in fresh water. In certain embodiments, the fourth aqueous mixture containing cations is sea water. The zwitterionic co-surfactant can contain cocamidopropyl hydroxysultaine. The petroleum sulfonate in the fluid containing nanosurfactants can range from 0.1 to 0.25 weight percent. The mineral oil in the fluid containing nanosurfactants can range from 0.002 to 0.02 weight percent. The zwitterionic co-surfactant in the fluid containing nanosurfactants can range from 0.1 to 0.2 weight percent.

Another method for producing nanosurfactant-containing fluids includes the steps of mixing a petroleum sulfonate surfactant, mineral oil, and a zwitterionic co-surfactant with fresh water in a reactor; introducing an aqueous salt solution to the reactor; and operating the reactor to produce nanoparticles containing the petroleum sulfonate surfactant, the mineral oil, and the zwitterionic co-surfactant and having a particle diameter ranging from about 10 nm to 100 nm. These nanoparticles are stable under high salinity and high temperature conditions. In certain embodiments, a substantial portion of the nanoparticles are stable for at least three months at temperatures of at least 100° C. The zwitterionic co-surfactant can contain cocamidopropyl hydroxysultaine.

Embodiments of the disclosure provide a method of producing a nanosurfactant composition suitable for injection into a hydrocarbon-bearing formation for enhanced oil recovery operations. The method includes the step of combining a sulfonate surfactant and an oil to form a first mixture. The method includes the step of combining the first mixture, a zwitterionic co-surfactant, and fresh water to form a second mixture. The method includes the step of combining the second mixture, a cationic co-surfactant, and the fresh water to form a third mixture. The method includes the step of combining the third mixture and saline water to form the nanosurfactant composition. The saline water includes divalent cations. The nanosurfactant composition includes a nanoassembly. The nanoassembly has a hydrophobic interior and a hydrophilic exterior. The hydrophobic interior includes the sulfonate surfactant, a hydrophobic portion of the zwitterionic co-surfactant, a hydrophobic portion of the cationic co-surfactant, and the oil. The hydrophilic exterior includes a hydrophilic portion of the zwitterionic co-surfactant and a hydrophilic portion of the cationic co-surfactant. The hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant stabilize the nanoassembly by interacting with the divalent cations present in the saline water. The oil facilitates containment of the sulfonate surfactant in the hydrophobic interior of the nanoassembly.

In some embodiments, the nanoassembly has a diameter ranging between 10 nm and 100 nm. In some embodiments, the sulfonate surfactant is petroleum sulfonate. In some embodiments, the petroleum sulfonate includes an alkyl sulfonate, and alkyl aryl sulfonate, and combinations thereof. In some embodiments, the nanosurfactant composition has a sulfonate surfactant content ranging between 0.1 wt. % and 0.25 wt. %. In some embodiments, the zwitterionic co-surfactant is cocamidopropyl hydroxysultaine. In some embodiments, the nanosurfactant composition has a zwitterionic co-surfactant content ranging between 0.1 wt. % and 0.2 wt. %. In some embodiments, the cationic co-surfactant is cocamidopropyl PG-dimonium chloride phosphate. In some embodiments, the nanosurfactant composition has a cationic co-surfactant content ranging between 0.005 wt. % and 0.1 wt. %. In some embodiments, the oil is mineral oil. In some embodiments, the nanosurfactant composition has an oil content ranging between 0.002 wt. % and 0.02 wt. %. In some embodiments, the fresh water has a total dissolved solids concentration less than 3,000 ppm. In some embodiments, the saline water has a total dissolved solids concentration greater than 10,000 ppm.

Embodiments of the disclosure also provide a nanosurfactant composition suitable for injection into a hydrocarbon-bearing formation for enhanced oil recovery operations. The nanosurfactant composition includes a nanoassembly and an aqueous environment. The nanoassembly includes a hydrophobic interior and a hydrophilic exterior. The hydrophobic interior includes petroleum sulfonate, a hydrophobic portion of a zwitterionic co-surfactant, a hydrophobic portion of a cationic co-surfactant, and mineral oil. The hydrophilic exterior includes a hydrophilic portion of the zwitterionic co-surfactant and a hydrophilic portion of the cationic co-surfactant. The aqueous environment includes divalent cations. The hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant are configured to stabilize the nanoassembly by interacting with the divalent cations present in the aqueous environment. The mineral oil is configured to facilitate containment of the petroleum sulfonate in the hydrophobic interior of the nanoassembly.

In some embodiments, the nanoassembly has a diameter ranging between 10 nm and 100 nm. In some embodiments, the zwitterionic co-surfactant is cocamidopropyl hydroxysultaine. In some embodiments, the cationic co-surfactant is cocamidopropyl PG-dimonium chloride phosphate.

Embodiments of the disclosure also provide a method for recovering hydrocarbons from a hydrocarbon-bearing formation. The method includes the step of introducing into the hydrocarbon-bearing formation a nanosurfactant composition.

The nanosurfactant composition is formed by the step of combining a sulfonate surfactant and an oil to form a first mixture. The nanosurfactant composition is formed by the step of combining the first mixture, a zwitterionic co-surfactant, and fresh water to form a second mixture. The nanosurfactant composition is formed by the step of combining the second mixture, a cationic co-surfactant, and the fresh water to form a third mixture. The nanosurfactant composition is formed by the step of combining the third mixture and saline water to form the nanosurfactant composition. The saline water includes divalent cations. The nanosurfactant composition includes a nanoassembly. The nanoassembly has a hydrophobic interior and a hydrophilic exterior. The hydrophobic interior comprising the sulfonate surfactant, a hydrophobic portion of the zwitterionic co-surfactant, a hydrophobic portion of the cationic co-surfactant, and the oil. The hydrophilic exterior includes a hydrophilic portion of the zwitterionic co-surfactant and a hydrophilic portion of the cationic co-surfactant. The hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant stabilize the nanoassembly by interacting with the divalent cations present in the saline water. The oil facilitates containment of the sulfonate surfactant in the hydrophobic interior of the nanoassembly. The method includes the step of driving the nanosurfactant composition through the hydrocarbon-bearing formation to displace the hydrocarbons from the hydrocarbon-bearing formation. The nanoassembly reduces interfacial tension between the hydrocarbons and the saline water.

In some embodiments, the sulfonate surfactant is petroleum sulfonate. In some embodiments, the zwitterionic co-surfactant is cocamidopropyl hydroxysultaine. In some embodiments, the cationic co-surfactant is cocamidopropyl PG-dimonium chloride phosphate. In some embodiments, the oil is mineral oil.

Embodiments of the disclosure also provide a method of producing a nanosurfactant composition suitable for injection into a hydrocarbon-bearing formation for enhanced oil recovery operations. The method includes the step of combining a sulfonate surfactant, an oil, and fresh water to form a first mixture. The method includes the step of combining a zwitterionic co-surfactant and the fresh water to form a second mixture. The method includes the step of combining a cationic co-surfactant and the fresh water to form a third mixture. The method includes the step of combining the first mixture, the second mixture, and the third mixture to form a fourth mixture. The method includes the step of combining the fourth mixture and saline water to form the nanosurfactant composition. The saline water includes divalent cations. The nanosurfactant composition includes a nanoassembly. The nanoassembly has a hydrophobic interior and a hydrophilic exterior. The hydrophobic interior includes the sulfonate surfactant, a hydrophobic portion of the zwitterionic co-surfactant, a hydrophobic portion of the cationic co-surfactant, and the oil. The hydrophilic exterior includes a hydrophilic portion of the zwitterionic co-surfactant and a hydrophilic portion of the cationic co-surfactant. The hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant stabilize the nanoassembly by interacting with the divalent cations present in the saline water. The oil facilitates containment of the sulfonate surfactant in the hydrophobic interior of the nanoassembly.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be readily understood by the following detailed description in conjunction with the accompanying drawings. Embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings.

FIG. 1 is a drawing showing how size exclusion and chromatographic effects enable nanosurfactants to reach the oil-water interfaces.

FIG. 2 shows photographs demonstrating the components used in the preparation of the nanosurfactant compositions, according to an embodiment.

FIG. 3 is a photograph of a cryo-transmission electron microscopy (cryo-TEM) image of the nanosurfactant, according to an embodiment.

FIGS. 4A and 4B are photographs of nanosurfactant-containing fluids after 4 months at room temperature and following incubation in the oven at 100° C., respectively, according to an embodiment.

FIG. 5 is a graphical representation of the particle size of the nanoparticles in nanosurfactant mixture during heating followed by cooling, according to an embodiment.

FIGS. 6A, 6B, and 6C are graphical representations of the particle size of the nanoparticles at different dilutions of the nanosurfactant mixture during heating followed by cooling, according to an embodiment.

FIG. 7 is a graphical representation of the results from the interfacial tension (IFT) evaluation of the zwitterionic co-surfactant alone (STRX) and when it is present as part of the nanosurfactant mixture (NS STRX), according to an embodiment.

FIG. 8 is a graphical representation of the results from the IFT evaluation of the nanosurfactant mixture before and after being maintained at 100° C. for over 4 months, according to an embodiment.

FIGS. 9A and 9B are graphical representations of the results from the IFT evaluation of different concentrations of the co-surfactant alone (STRX) and when present as part of the nanosurfactant mixture (NS STRX), according to an embodiment.

FIG. 10 is a photograph showing the various nanosurfactant samples with different ratios of the zwitterionic co-surfactant to petroleum sulfonate. FIG. 10 shows seven samples whose labeling correspond to sample numbers provided in Table 6.

FIG. 11 is a graphical representation of the results from the IFT evaluation of a nanosurfactant mixture with mineral oil and nanosurfactant mixtures without the mineral oil.

FIG. 12 is a graphical representation of the results from the IFT evaluation of the supernatant, which was obtained following filtration to remove the precipitate formed by the reaction between seawater and the petroleum sulfonate surfactant.

FIGS. 13A-13E are photographs showing the various steps of the preparation of a soluble fraction of petroleum sulfonate mixed with sea water, according to an embodiment.

FIG. 14 is a graphical representation of the results obtained following interfacial tension evaluation of the seawater alone (shown as blue line labeled SW on the graph) and the nanosurfactant mixture with petroleum sulfonate surfactant, zwitterionic co-surfactant, and mineral oil mixed with sea water (shown as green line labeled STRX on the graph).

FIG. 15 is a schematic illustration for the phase behavior experimental setup, according to an embodiment.

FIGS. 16A-16D are photographic images under normal light and under ultraviolet (UV) light of the zwitterionic co-surfactant alone and the nanosurfactant mixture with zwitterionic co-surfactant in contact with crude oil; and both mixtures being incubated at 100° C. for one week.

FIG. 17 is a schematic illustration for the phase behavior experimental setup, according to an embodiment.

FIGS. 18A and 18C are photographic images under normal light and FIGS. 18B and 18D are photographic images under UV light of the seawater alone (SW) and the nanosurfactant mixture with a zwitterionic co-surfactant (STRX), all samples being incubated with a second layer of a mixture of petroleum sulfonate and oil. FIGS. 18A and 18B are photographs of the samples taken before heating and FIGS. 18C and 18D are photographs of the samples taken after heating at 100° C. for 1 hour.

FIG. 19A is a proton nuclear magnetic resonance (¹H-NMR) spectrum of the nanosurfactant mixture and each of its individual ingredients. Regions of the spectrum in FIG. 19A that are highlighted as a red rectangle and a green rectangle are enhanced and shown separately in FIGS. 19B and 19C, respectively.

FIGS. 20A and 20B are ¹H-NMR spectra of samples of the nanosurfactant-containing fluid collected before and after contact with powdered Arab-D outcrop rock, respectively, according to an embodiment.

FIGS. 21A and 21B are ¹H-NMR spectra of samples of a fluid containing the zwitterionic co-surfactant collected before and after contact with powdered Arab-D outcrop rock, respectively, according to an embodiment.

FIG. 22 is a series of difference spectra based on the ¹H-NMR spectra of samples of a fluid containing the zwitterionic co-surfactant collected before and after contact with powdered Arab-D outcrop rock, respectively, according to an embodiment.

FIG. 23 is a difference spectrum based on the ¹H-NMR spectra of samples of the nanosurfactant-containing fluid collected before and after contact with powdered Arab-D outcrop rock, respectively, according to an embodiment.

FIG. 24 is a graphical representation of the adsorption of active ingredients onto rock from different compositions—the zwitterionic co-surfactant alone (STRX control), the petroleum sulfonate alone (NS EOR-2095), the nanosurfactant mixture with the petroleum sulfonate and the zwitterionic co-surfactant (NS STRX), and the nanosurfactant mixture with the petroleum sulfonate, the zwitterionic co-surfactant, and mineral oil (NS min oil), according to an embodiment.

FIG. 25 is a graphical representation of the active ingredients that remain in solution from different compositions—the zwitterionic co-surfactant alone (STRX control), the petroleum sulfonate alone (NS EOR-2095), the nanosurfactant mixture with the petroleum sulfonate and the zwitterionic co-surfactant (NS STRX), and the nanosurfactant mixture with the petroleum sulfonate, the zwitterionic co-surfactant, and mineral oil (NS min oil), according to an embodiment.

FIGS. 26A and 26B are photographs of test tubes containing compositions of the nanosurfactant mixture with the petroleum sulfonate, the zwitterionic co-surfactant, and mineral oil, before and after a seven day incubation period. FIG. 26C is a graphical representation of the results from an IFT evaluation of the nanosurfactant mixture with the petroleum sulfonate, the zwitterionic co-surfactant, and mineral oil.

FIG. 27 is a diagrammatic representation of the composite arrangement of core plugs for the coreflooding experiments, according to an embodiment.

FIG. 28 is a schematic illustration of the automatic coreflooding system, according to an embodiment.

FIG. 29A is a graphical representation of the results from the coreflooding recovery experiment using a nanosurfactant mixture, according to an embodiment; FIG. 29B is a zoomed-in version of a select section of FIG. 29A.

FIG. 30 is a schematic illustration of a nanoassembly included in a nanosurfactant composition, according to an embodiment.

FIG. 31A is a graphical representation of a breakthrough curve of a core flooding experiment using a sample nanosurfactant composition, according to an embodiment. FIG. 31B is a graphical representation of a breakthrough curve of a core flooding experiment using a control nanosurfactant composition, according to an embodiment.

FIG. 32 is a graphical representation showing the dynamic retention of the nanosurfactant composition the sample nanosurfactant composition and the control nanosurfactant composition, according to an embodiment.

DETAILED DESCRIPTION

Embodiments of this disclosure describe nanoparticle compositions that are stable under high salinity and high temperature conditions. More specifically, these compositions include petroleum sulfonate-based nanoparticles that are used for improved and enhanced oil recovery applications.

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments and reference is made to the accompanying drawings that form a part hereof. In other instances, well-known processes and methods may not be described in particular detail to refrain from unnecessarily obscuring the embodiments described here. Additionally, illustrations of embodiments here may omit certain features and/or details in order to not obscure the embodiments described here. Other embodiments may be utilized, and logical changes may be made without departing from the scope of the disclosure. Therefore, the following detailed description is not to be taken in a limiting sense.

The description may use the phrases “in certain embodiments,” “in an embodiment,” or “in embodiments,” which may each refer to one or more of the same or different embodiments. Furthermore, the terms “comprising,” “including,” “having,” and the like, as used with respect to embodiments of the present disclosure, are synonymous. As used here, the term “effective amount” refers to at least that amount of nanosurfactant or nanosurfactant components necessary to bring about a desired result, such as, for example, enhanced oil recovery or improved stability at high temperatures or improved stability for longer periods of time and at relatively high temperatures. The term salinity refers to the amount of total dissolved solids (TDS) in the water and is measured in parts per million (ppm). Water with a TDS concentration less than 3,000 ppm is considered fresh water. Water with a TDS concentration in excess of 10,000 ppm is considered saline. The term “high salinity conditions” refers to fluid conditions where the TDS concentration ranges from 30,000 ppm to 220,000 ppm. In certain embodiments, high salinity conditions include fluid conditions with the TDS concentration ranging from 60,000 ppm to 150,000 ppm. The term “high temperature conditions” refers to fluid or reservoir conditions where the temperature ranges from 75° C. to 150° C. In certain embodiments, high temperature conditions include fluid or reservoir conditions with the temperature ranging from 100° C. to 120° C.

Embodiments include nanosurfactant formulations and use of these formulations with long-term stability at high salinity and high temperature conditions. Nanosurfactants described here are nanoparticle compositions containing a sulfonate surfactant, a zwitterionic co-surfactant, and an oil. The nanosurfactants enable more economical oil recovery as compared to conventional surfactants by reducing the amount of surfactants lost by adsorption onto the rock surfaces. These compositions deliver surfactants more efficiently to the oil-water interfaces. Embodiments include aqueous suspensions of petroleum sulfonate-based nanoparticles with long-term stability at high salinity and high temperature conditions. These formulations contain small amounts of a class of zwitterionic co-surfactants (a surfactant with both anionic and cationic centers in the same molecule) that have no easily hydrolysable chemical bonds. These formulations are compatible with salty and hard water, in particular tolerant to high concentrations of sodium chloride and divalent cations. An example of a commercially-available zwitterionic surfactants used in these formulations is cocamidopropyl hydroxysultaine or betaine surfactants, such as SURFATEX CBS™, obtained from Surfactants International, LLC, headquartered in Allendale, N.J., USA; PETROSTEP® SB, PETROSTEP® CG-50, and Amphosol® CG-50 from Stepan, headquartered in Northfield, Ill., USA; and ColaTeric CBS-HP from Colonial Chemical Inc., headquartered in South Pittsburgh, Tenn. These formulations offer several advantages, such as colloidal and chemical stability at high salinity and high temperature for several months, substantial reduction of crude oil/seawater interfacial tension, ability to form an emulsion very rapidly upon contact with crude oil without any mechanical mixing. As demonstrated by experimental data from a lab-scale coreflooding setting, these formulations show appreciable increase in oil recovery beyond seawater flooding.

The formulations described here include sulfonates, mineral oil, and a class of co-surfactants that have no easily hydrolysable chemical bonds. For example, a nanosurfactant mixture was formulated using zwitterionic co-surfactants with petroleum sulfonate surfactants in the presence of mineral oil. The term “petroleum sulfonate” refers to a mixture containing sulfonated benzenoids (both alkyl aryl and aryl), and cycloaliphatic and paraffinic (alkyl) hydrocarbons in various ratios to one another depending on the nature of the source of the petroleum fraction. Petroleum sulfonates can include alkyl xylene sulfonates, alkyl aryl sulfonates, alpha-olefin sulfonates, and combinations thereof. An example of a commercially-available product that contains petroleum sulfonate is PETRONATE® EOR-2095 sodium sulfonate (EOR-2095) from Chemtura Corporation (headquartered in Philadelphia, Pa., USA) or PETRONATE® sodium sulfonates from Sonneborn LLC (headquartered in Parsippany, N.J., USA). Petroleum sulfonates are not stable by themselves in sea water. But, the formulation of zwitterionic co-surfactants with petroleum sulfonate surfactants in the presence of mineral oil was successfully operative in long-term stability tests in seawater and low salinity Arab-D water at 100° C. Seawater used in the experiments disclosed here has a TDS concentration of about 60,000 ppm. Low salinity Arab-D water has a TDS concentration of about 120,000 ppm. Certain alcohol ether sulfonates are not suitable for embodiments disclosed here as alcohol ether sulfonates do not combine with the zwitterionic co-surfactants to form the nanoparticles for cross well applications. Additional hydroxyl groups presented by the alcohol ether sulfonates increase material losses as they bind to calcium. In EOR fluids with seawater, the alcohol ether sulfonates bind preferentially to the carbonate rock instead of remaining available in the EOR fluid.

Embodiments disclosed here include compositions suitable for injection into a hydrocarbon-bearing formation for enhanced recovery operations. One such composition includes a sulfonate surfactant, a zwitterionic co-surfactant, and mineral oil.

In certain embodiments, the effective amounts of each of the sulfonate surfactant and the zwitterionic co-surfactant range from 0.1 to 0.9 wt % of the nanosurfactant mixture. In certain embodiments, the effective amounts of each of the sulfonate surfactant and the zwitterionic co-surfactant range from 0.1 to 0.5 wt % of the nanosurfactant mixture. In certain embodiments, the total amount of the sulfonate surfactant and the zwitterionic co-surfactant ranges from 0.2 to 1 wt % of the nanosurfactant mixture. In certain embodiments, the amount of mineral oil ranges from 0.002 wt %-0.02 wt % of the nanosurfactant mixture. In certain embodiments, the amount of mineral oil ranges from 0.002 wt %-0.01 wt % of the nanosurfactant mixture. In certain embodiments, the amount of mineral oil is approximately 0.005 wt % of the nanosurfactant mixture.

The wt % ratio of sulfonate/zwitterionic surfactants can range from 0.3 to 3. In certain embodiments, the wt % ratio of sulfonates/zwitterionic surfactants can range from 0.3 to 2.5. In certain embodiments, the wt % ratio of sulfonates/zwitterionic surfactants can range from 0.3 to 2.0. In certain embodiments, the wt % ratio of sulfonates/zwitterionic surfactants can range from 0.3 to 1.5. In certain embodiments, the wt % ratio of sulfonates/zwitterionic surfactants can range from 0.3 to 1.0. In certain embodiments, the wt % ratio of sulfonates/zwitterionic surfactants can range from 0.5 to 1.5. In certain embodiments, the wt % ratio of sulfonates/zwitterionic surfactants can range from 0.5 to 1. In certain embodiments, the wt % ratio of sulfonates/zwitterionic surfactants can range from 0.5 to 0.8. In certain embodiments, the wt % ratio of sulfonates/zwitterionic surfactants can range from 0.75 to 0.80.

Embodiments disclosed here include methods for recovering hydrocarbons from a hydrocarbon-bearing formation. One such method includes introducing into the hydrocarbon-bearing formation a fluid containing effective amounts of a sulfonate surfactant, a zwitterionic co-surfactant, and mineral oil; driving the fluid through the hydrocarbon-bearing formation to displace hydrocarbons from the hydrocarbon-bearing formation; and recovering the displaced hydrocarbons. The sulfonate surfactant and the zwitterionic co-surfactant can form nanoparticles having a particle diameter ranging from about 10 nm to 100 nm. In certain embodiments, the petroleum sulfonate-based nanoparticles in seawater-based formulations have particle diameters ranging from about 10 nm to 60 nm.

In an embodiment, a zwitterionic co-surfactant, such as cocamidopropyl hydroxysultaine, a petroleum sulfonate surfactant, such as sodium alkylbenzene sulfonates, and seawater were combined to form a colloidally and chemically stable formulation. Certain embodiments of these nanosurfactant formulations were colloidally and chemically stable for more than six months at 100° C. Stability refers to the ability of the particles to remain as part of the nanosurfactant composition without aggregating or displaying reduced sticking to the rock surfaces. Stability does not refer to the stability of the individual components. In certain embodiments, the seawater-crude oil IFT was reduced by two to three orders of magnitude. Certain embodiments showed reduced interfacial tension measured in milliNewtons per meter (mN/m) with crude oil at 90° C. and rapid formation of an emulsion at 100° C. without any mechanical mixing. Furthermore, in certain embodiments, the size of the formed nanoparticles was small (less than 50 nm) in comparison to pore diameters typically encountered in petroleum-bearing carbonate formations. The size of the nanoparticles was small and decreased to about 15 nm after increasing the temperature from 25° C. to 90° C. The size remained unchanged when the suspension is cooled back to 25° C. This indicates the enhanced stability of the formulation under oil reservoir conditions, and even when different temperatures are encountered in the reservoir.

When the surfactant molecules are formulated into nano-sized particles, the loss of surfactants is mitigated and the delivery of surfactants to the oil phase is enhanced. As illustrated in FIG. 1, due to size or volume exclusion as well as chromatographic effects, nano-sized particles can migrate long distances and efficiently deliver the surfactant to the entrapped or upswept oil for enhanced mobility. Petroleum sulfonate surfactants are inexpensive, readily available surfactants, and provide an exceptional performance for oil recovery applications. However, the sulfonates are only soluble in fresh water, and they form insoluble gummy precipitates in seawater. The precipitates consist of petroleum sulfonate salts of the naturally occurring divalent metal cations in seawater. These petroleum sulfonate salts are not very soluble in water, so most of the surfactant is kept in a solid form along with the metal cations, and only a small amount of the salts dissolves in seawater. The solid particles gradually dissolve in water when the already dissolved surfactant is consumed by oil.

When delivered as part of the nanoparticle composition, the surfactant is released in the presence of oil, otherwise it remains intact within the water phase. This approach is efficient and economical in delivering surfactants in targeted oil recovery applications. The nanoparticles are small enough to travel through the reservoir without straining. The sorption of these particles onto the rock matrix is not significant, and they are stable for periods of time exceeding their travel time to the oil-water interface. Flow of these aqueous nanoparticle compositions through the reservoir is different from the flow of foam compositions. These aqueous nanoparticle compositions easily permeate into low permeability zones, while the foam compositions do not have similar capability. Moreover, these aqueous nanoparticle compositions do not block the high permeability zones of a reservoir.

Previous surfactant formulations showed stability of about three to four days, and were thus deemed unsuitable for enhanced oil recovery applications. These prior surfactant formulations revealed instability and phase separation in the concentrated and seawater-diluted nanosurfactant suspensions even at room temperature (23-27° C.). Transmission electron microscopy (TEM) imaging indicated that the formed particles were present both as individual particles as well as linear (chain-like) aggregates up to 200 nm long. Both concentrated and seawater-diluted suspensions became more unstable at elevated temperatures (˜100° C.). Phase separation and aggregation were apparent after less than three days of exposure to 100° C. Noticeable precipitation was observed at greater seawater dilutions while separation of the oily petroleum sulfonate salts was apparent at lower dilutions. Samples with a median dilution showed a crossover between petroleum sulfonate separation and precipitation. Results also implied noticeable sorption of nanosurfactant components onto crushed rock grains at elevated temperatures and the formation of a significant number of aggregates in the supernatant after sorption.

Embodiments formulated using petroleum sulfonate surfactants, zwitterionic co-surfactants, and mineral oil with did not suffer from the disadvantages described in the prior paragraph. Due to the co-existence of immiscible components (oil and water) in the nanosurfactant solution, the configuration of nanosurfactants formed in seawater is fundamentally different than just a mixture (or a blend) of surfactant molecules. The nanosurfactants are emulsions of nano-sized oil droplets that contain the petroleum sulfonate. The droplets are in turn stabilized by the co-surfactant molecules, which are not easily hydrolyzed in seawater.

Embodiments disclosed here include methods for producing nanosurfactant compositions suitable for injection into a hydrocarbon-bearing formation for enhanced recovery operations. One such method includes the steps of mixing a sulfonate surfactant and a zwitterionic co-surfactant in the presence of freshwater or deionized water. In certain embodiments, the sulfonate surfactant contains 2 wt % to 20 wt % of mineral oil. If the sulfonate surfactant does not contain the required amount of mineral oil, then external mineral oil is added to the sulfonate surfactant. If required, the various fluids are diluted with freshwater or deionized water to the desired concentrations. The method further includes introducing an aqueous salt solution with cations to the reactor to the freshwater mixture of sulfonate surfactant and zwitterionic co-surfactant to produce a nanosurfactant composition containing nanoparticles with a particle diameter in a range of from about 10 nm to 100 nm and the sulfonate surfactant and the zwitterionic co-surfactant at about 0.2-1 wt %. These nanoparticles are stable under high salinity and high temperature conditions. A substantial portion of the nanoparticles is stable for at least three months at temperatures of at least 100° C. The sulfonate surfactant is selected from the group consisting of an alkyl sulfonate, an alkyl aryl sulfonate, and combinations thereof. In certain embodiments, the sulfonate surfactant is a petroleum sulfonate salt. In certain embodiments, the petroleum sulfonate nanoparticles in seawater-based formulations have particle diameters ranging from about 10 nm to 60 nm. In certain embodiments, the zwitterionic co-surfactant contains cocamidopropyl hydroxysultaine.

Another method of producing nanosurfactant compositions includes mixing the sulfonate surfactant with the zwitterionic co-surfactant in the presence of fresh water. The resulting mixture contains water in an amount ranging from 80 wt % to 98 wt % and the total amount of surfactants ranging from about 2 wt % to 20 wt %. After mixing the surfactants, water containing cations, such as seawater, is introduced to form nanosurfactant compositions with the total amount of surfactants ranging from about 0.2 wt % to 1 wt %. The cations include one or more of sodium, calcium, magnesium, and potassium. The cations-containing water has a TDS concentration ranging from 50,000 ppm-150,000 ppm. In certain embodiments, the nanosurfactant compositions contain 0.1 wt %-0.25 wt % of petroleum sulfonates, 0.002 wt %-0.02 wt % of mineral oil, and 0.1 wt %-0.2 wt % of a zwitterionic co-surfactant. These nanosurfactant compositions are stable under reservoir conditions. An example of reservoir conditions includes a TDS concentration ranging from 60,000 ppm to 150,000 ppm and a reservoir temperature ranging from 100° C. to 120° C.

These nanosurfactant compositions are different from the fracturing fluids that contain zwitterionic and ionic surfactants. The fracturing fluids contain 10 wt % to 20 wt % of the zwitterionic surfactants and the methods of production of these fracturing fluids do not permit the formation of the nanoparticles. As the fracturing fluids contain large amounts of surfactants to increase fluid viscosity, the fracturing fluids do not pass through the pores but instead fracture the rocks of the reservoir. Disclosed here are nanosurfactant compositions containing surfactants ranging from about 0.2 wt % to 1 wt % of the fluid used for improved or enhanced oil recovery. At this reduced concentration of surfactants, the fluid containing the nanosurfactants passes through the pores. The salts contained in the seawater force the surfactant and co-surfactant to form nanoparticles and stabilize these nanoparticles. The fluid containing the nanoparticles passes through the pores easily and delivers the active surfactants to mobilize oil and enhance recovery.

In some embodiments, the nanosurfactant composition includes oil-containing nanoassemblies. As shown in FIG. 30, the nanoassembly is essentially an oil-based micelle surrounded by the zwitterionic co-surfactant (the hydrophilic heads of the zwitterionic co-surfactant are represented by the bright spheres). The nanoassembly includes a hydrophobic interior and a hydrophilic exterior. The hydrophobic interior includes the mineral oil. The hydrophobic interior also includes the hydrophobic portion of the sulfonate surfactant and the hydrophobic portion of the zwitterionic co-surfactant. The hydrophilic portion of the zwitterionic co-surfactant forming the hydrophilic exterior of the micelle interacts with the saline aqueous environment to stabilize the nanoassembly. Specifically, the hydrophilic portion of the zwitterionic co-surfactant interacts with the divalent cations present in the saline aqueous environment. In some embodiments, the sulfonate surfactant is contained within the hydrophobic interior. In alternate embodiments, the hydrophilic portion of the sulfonate surfactant (represented by the dark sphere) is shielded by adjacent hydrophilic portions of the zwitterionic co-surfactant. In this manner, the hydrophilic portion of the sulfonate surfactant, which may be due to steric hinderance by the hydrophilic portion of the zwitterionic co-surfactant, sparingly or does not participate in the stabilization of the nanoassembly with the exterior saline aqueous environment. In some embodiments, the mineral oil in the hydrophobic interior facilitates the containment of the sulfonate surfactant in the hydrophobic interior of the nanoassembly.

Embodiments provide nanosurfactant compositions suitable for injection into a hydrocarbon-bearing formation for enhanced recovery operations. A non-limiting example nanosurfactant composition includes a sulfonate surfactant, a zwitterionic co-surfactant, a cationic co-surfactant, mineral oil, and saline water.

In some embodiments, the nanosurfactant composition has a sulfonate surfactant content ranging between about 0.1 wt. % and about 0.25 wt. %. In some embodiments, the nanosurfactant composition has a zwitterionic co-surfactant content ranging between about 0.1 wt. % and about 0,2 wt. %. In some embodiments, the nanosurfactant composition has a cationic co-surfactant content ranging between about 0.005 wt. % and about 0.1 wt. %. In some embodiments, the nanosurfactant composition has an oil content ranging between about 0.002 wt. % and about 0.02 wt. %.

Embodiments include nanosurfactant compositions and use of these compositions with long-term stability at high salinity and high temperature conditions. The nanosurfactant compositions include nanoassemblies including a sulfonate surfactant, a zwitterionic co-surfactant, a cationic co-surfactant, and an oil (such as mineral oil). The hydrophobic interior of the nanoassembly includes the mineral oil, the hydrophobic portion of the sulfonate surfactant, the hydrophobic portion of the zwitterionic co-surfactant, and the hydrophobic portion of the cationic co-surfactant. In some embodiments, the hydrophilic portion of the sulfonate surfactant is located in the hydrophobic interior of the nanoassembly. The hydrophilic exterior of the nanoassembly includes the hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant. Such nanosurfactant compositions are capable of reducing the overall dynamic retention on carbonate rock by greater than about 50% compared to nanosurfactant compositions that are in the absence of the cationic co-surfactant. The zwitterionic co-surfactant is hydrolysis resistant.

In some embodiments, the sulfonate surfactant can include petroleum sulfonate. Petroleum sulfonate can include alkyl sulfonates, alkyl aryl sulfonate, alkyl xylene sulfonate, and alpha-olefin sulfonates, and combinations of the same. Non-limiting examples of a commercially-available product that contains petroleum sulfonate include PETRONATE® EOR-2095 sodium sulfonates and PETRONATE® HL/L sodium sulfonates.

In some embodiments, the zwitterionic co-surfactant can include cocamidopropyl hydroxysultaine or betaine-based surfactants. The molecular structure of cocamidopropyl hydroxysultaine (or N,N-dimethyl-N-(3-cocamidopropyl)-3-ammonio-2-hydroxypropylsulfonate) is shown in Formula (I):

where n is a natural number ranging from 1 to 20. Non-limiting examples of a commercially available product that contains cocamidopropyl hydroxysultaine or betaine-based surfactants include SURFATEX CBS™, PETROSTEP® SB, PETROSTEP® CG-50, Amphosol® CG-50, and ColaTeric CBS-HP.

In some embodiments, the cationic co-surfactant can include cocamidopropyl PG-dimonium chloride phosphate. The cationic co-surfactant can include the cationic counterpart of cocamidopropyl PG-dimonium chloride phosphate. The molecular structure of cocamidopropyl PG-dimonium chloride phosphate (or mono-, bis-, or tris(N-C5-17-alkylamidopropyl-N,N-demethyl-N-(2-hydroxypropyl)ammonio)phosphate trichloride) is shown in Formula (II):

where n is a natural number ranging from 1 to 20, and x and y are natural numbers where the sum of x and y is three.

Generally speaking, the combination of a two or more oppositely-charged surfactants in a certain surfactant formulation may lead to severe aggregation or precipitation in aqueous phase, which makes such surfactant formulation unsuitable for injection into a hydrocarbon-bearing formation. Advantageously, embodiments of the disclosure provide a method to form a colloidally stable water-based nanosurfactant composition having two or more oppositely-charged surfactants in a controlled manner that does not result in aggregation or precipitation.

In an example embodiment of the method, the petroleum sulfonate and the mineral oil are combined to form a first mixture. Subsequently, the first mixture, the zwitterionic co-surfactant, and fresh water are combined to form a second mixture. The second mixture, the cationic co-surfactant, and additional fresh water are combined to form a third mixture. Thereafter, the third mixture and saline water are combined to form the nanosurfactant composition. The saline water includes divalent cations where the hydrophilic position of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant interact with the divalent cations present in the aqueous environment to stabilize the nanoassembly. The mineral oil facilitates the containment of the petroleum sulfonate in the hydrophobic interior of the nanoassembly.

In an example embodiment of the method, the petroleum sulfonate, the mineral oil, a do fresh water are combined to form a first mixture. The zwitterionic co-surfactant and fresh water are combined to form a second mixture. The cationic co-surfactant and fresh water are combined to form a third mixture. Subsequently, the first mixture, the second mixture, and the third mixture are combined to form a fourth mixture. Thereafter, the fourth mixture and saline water are combined to form the nanosurfactant composition. The saline water includes divalent cations where the hydrophilic position of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant interact with the divalent cations present in the aqueous environment to stabilize the nanoassembly. The mineral oil facilitates the containment of the petroleum sulfonate in the hydrophobic interior of the nanoassembly.

In an example embodiment of the method, the nanosurfactant composition is introduced into a hydrocarbon-bearing formation. The hydrocarbon-bearing formation can include carbonate-based rocks. The nanosurfactant composition is driven through the hydrocarbon-bearing formation such that the nanoassembly makes contact with and displaces hydrocarbons from the formation by reducing the interfacial tension between the hydrocarbons and the saline water included in the nanosurfactant composition.

EXAMPLES

Examples of certain embodiments are provided here to facilitate a better understanding of the nanosurfactant compositions and methods of production of the nano surfactant compositions for use in enhanced oil recovery.

Example 1

Experiments were conducted to evaluate performance of a petroleum sulfonate surfactant (EOR-2095) in combination with specific zwitterionic co-surfactants. Four zwitterionic co-surfactants were tested. Amphosol® LB is a mild amphoteric surfactant, containing lauramidopropyl betaine & myristamidopropyl betaine. PETROSTEP® CG-50 contains cocamidopropyl betaines. PETROSTEP® SB contains cocoamidopropyl hydroxysultaine. SURFATEX CBS™ contains cocamidopropyl hydroxysultaine. The qualitative results of these zwitterionic co-surfactants with EOR-2095 were analyzed by stability tests. Two co-surfactants—PETROSTEP® SB and SURFATEX® CBS (STRX)—were chosen for further experimentation with different sulfonates (EOR-095, BIOSOFT S101®, NACCANOL 90G®, G-3300®, ENORDET O342®, ENORDET O352®, ENORDET O242®). These surfactant and co-surfactant formulations were evalutated in seawater as well as in low salinity Arab-D brine. The stability and properties of the nanosurfactant compositions are dependent on type of the sulfonates used as the core of the nanostructured entity. The properties of the nanosurfactants are affected by factors such as co-surfactant type, salt concentration, type of petroleum sulfonate/alternative, oil content, and amount. Based on these tests, EOR-2095 and SURFATEX® CBS were selected for conducting further analysis.

Example 2

Provided below in Table 1 is an example of the composition of the synthetic seawater. The different compounds were added in grams as shown in Table 1 to make one liter of synthetic seawater.

TABLE 1 NaCl CaCl₂•2H₂O MgCl₂•6H₂O Na₂SO₄ NaHCO₃ 41.04 2.384 17.645 6.343 0.165

FIG. 2 shows an example of a method for the preparation of the nanosurfactant mixture. Provided is an example of a process for the preparation of nanosurfactant (stock solution) using a zwitterionic co-surfactant and petroleum sulfonate with mineral oil. A 5% stock solution of EOR-2095 surfactant was prepared by dissolving commercial 50 g of EOR-2095 in 900 milliliters (mL) of deionized water and adjusting the volume to 1000 mL with more deionized water once the dissolution is complete. A 4% stock solution of the zwitterionic co-surfactant was prepared by dissolving 40 g of the STRX commercial co-surfactant in 900 mL of deionized water and adjusting the volume to 1000 mL with more deionized water once the dissolution is complete. About 100 mL of the 5% EOR-2095 stock and 125 mL of the 4% co-surfactant stock were mixed and 1000 mL of synthetic seawater was added followed by vigorous mixing. The nanosurfactant mixture does not include any hydrolyzed polyacrylamides. The ratio of petroleum sulfonate to the zwitterionic co-surfactant can be varied and optimized to meet the desired properties of the final product.

Cryo-TEM was used to study the morphology of the nanosurfactant samples. About 20 microliters (μL) of the nanosurfactant mixture samples were deposited without dilution onto copper C-flat holey carbon grids (Product code: CF-1.2/1.3-4C-T-50 from Electron Microscopy Sciences). The samples were blotted and frozen on a Gatan CP3 Cryoplunge in liquid ethane cooled with liquid nitrogen. Samples were mounted on the autoloader of an FEI Tecnai Arctica Field Emission Cryo-TEM (available at Center of Nanoscale Systems, Harvard University, Cambridge, Mass., USA). Low electron dose images were taken under 200 kilovolts accelerating voltage. As shown in FIG. 3, spherical particles with dimeters ranging from 15 to 40 nm were observed for the nanosurfactant fluid. This result confirmed the size of the nanosurfactant particles being in the range required for better transportability in tight reservoir rocks.

The stability of the nanosurfactant suspensions was tested in seawater at elevated temperatures (100° C.) for more than four months. Nanosurfactant samples were placed in cylindrical pressure tubes with air-tight Teflon lids. The tubes were sealed tightly and incubated in the oven at 100° C. The stability of these samples was checked and photographed over different periods of time. FIG. 4A shows the nanosurfactant suspension after being stored for 4 months at room temperature and FIG. 4B shows the nanosurfactant suspension following incubation in the oven at 100° C. after 4 months. The nanosurfactant suspensions were still as stable as the suspensions at room temperature, as seen by the lack of phase separation. The color and turbidity of the oven-incubated and room-temperature samples did not change significantly.

Dynamic Light Scattering (DLS) analysis was used to measure changes in size of the nanosurfactant particles and aggregation behavior with increasing temperature and upon cooling back to room temperature. Different dilutions of about 3 mL nanosurfactant solutions in seawater (none, 1:3, 1:5, 1:7, 1:15) were placed in capped quartz cuvettes. The temperature was increased uniformly from 25° C. to 90° C. at 5° C. increments with a 15-minute equilibration time at each temperature prior to measuring the particle size. For each temperature, the particle size was taken as an average of five measurements of 90 seconds each. At the end of the heating cycle, measurements were repeated for the cooling process from 90° C. to 25° C. with all other parameters being the same. FIG. 5 is a graphical representation of the particle size of the nanosurfactant particles with the petroleum sulfonate and the zwitterionic co-surfactant as measured during the heating and cooling cycles. The size of the nanosurfactant particles with the zwitterionic co-surfactant decreases with increasing temperature and remains small after cooling. Similar behavior was observed at different seawater dilutions as shown in FIGS. 6A-6C. FIGS. 6A-6C are graphical representations of the particle size of the nanosurfactant particles when diluted with seawater as measured during the heating and cooling cycles. FIG. 6A shows the particle size of the nanosurfactant particles in the fluid that was not diluted with any further seawater. FIG. 6B shows the particle size of the nanosurfactant particles in the fluid that was diluted with seven parts of seawater and FIG. 6C shows the particle size of the nanosurfactant particles in the fluid that was diluted with fifteen parts of seawater. These observations represent interesting characteristics of this nanosurfactant formulation. The nanosurfactant particles reach smaller sizes when injected into the reservoir, and will remain small even if they experience lower temperatures during their journey within the reservoir.

Example 3—Interfacial Tension (IFT) Measurements

One of the most important characteristics that determine the efficiency of a surfactant treatment in EOR is the IFT reduction. The IFT between crude oil and an aqueous solution (i.e. nanosurfactant-containing fluid) was measured using a spinning drop interfacial tensiometer (M6500). The solution to be tested was filled in a capillary tube and a drop of filtered UTMN crude oil (from Uthmaniyah oil field) was spun at ˜4000 revolutions per minute (rpm) at 90° C. The diameter of the oil droplet was recorded every 5 minutes for around 30 minutes and used to calculate the IFT based on known density deference, temperature, speed, and the drop diameter. The IFT is calculated as:

$I\; F\; T\mspace{11mu}{\left( \frac{mN}{m} \right) = {{2.7}8 \times 10^{- 16}\frac{\pi^{2}}{8n^{3}}\left( {\rho_{a} - \rho_{o}} \right)\omega D^{3}}}$ ρ_(a)=density of the aqueous phase in grams per cubic centimeters (g/cm³) ρ_(o)=density of oil in g/cm³ D=diameter of cylindrical droplet in micrometers ω=rotation speed (rpm) n=refractive index of the aqueous solution

IFT Measurements for Nanosurfactant Samples

The IFT was measured for nanosurfactant samples and for fluids containing the zwitterionic co-surfactant alone. Table 2 and FIG. 7 show the IFT results for the nanosurfactant samples and for the zwitterionic co-surfactant alone. Extremely low IFT was observed for nanosurfactant sample as compared to the fluid containing the zwitterionic co-surfactant alone. These results signify that the reduction in IFT is mainly due to the cumulative functionality of the petroleum surfactant, the zwitterionic co-surfactant, and the mineral oil. This significant reduction of IFT results in better capillary action, and thus better mobilization of oil by the flood fluids compared to conventional surfactants. The results also signify the key role of petroleum sulfonates, which could only be made stable in seawater via transformation into nanosurfactants, and consequently used in oil recovery applications at high temperature and elevated salinities.

TABLE 2 Time IFT, mN/m (min) STRX-NS STRX  0 0.041 0.885  5 0.039 0.997 10 0.049 1.018 15 0.052 1.086 20 0.050 1.052 25 0.056 1.107 30 0.056 1.113

Additionally, interfacial tension evaluation was conducted after the nanosurfactant composition was kept in the oven for more than 4 months at 100° C. About 5 mL of the sample was taken from the tube and the rest of the sample was sealed tightly and returned to the oven. The results were compared with the IFT values for the same sample that was measured previously before being incubated in the oven. Table 3 and FIG. 8 show the IFT results for the nanosurfactant composition before (two independent runs) and after being in the oven at 100° C. for more than four months. As mentioned previously, the stability experiments revealed that the nanosurfactant composition was stable during this period. The IFT values of the nanosurfactant composition are almost identical (within acceptable measurements error), confirming the long-term functionality and thermal stability of the nanosurfactant composition.

TABLE 3 IFT Time STRX-NS STRX-NS STRX-NS after more (min) (Run 1) (Run 2) than 4 months at 100° C.  0 0.0599 0.0407 0.0484  5 0.0583 0.0392 0.0463 10 0.0604 0.0487 0.0472 15 0.0664 0.0515 0.0457 20 0.0658 0.0501 0.0424 25 0.0614 0.0558 0.0466 30 0.0720 0.0558 0.0440

More IFT evaluations were conducted using different dilutions of the zwitterionic co-surfactant alone and the nanosurfactant composition. These samples were prepared by mixing components as shown in Table 4, and their IFT was measured. For the nanosurfactant composition, the previously prepared stock solution was diluted with seawater and used. On the other hand, for the zwitterionic co-surfactant samples, a new solution was prepared by adding 10 mL deionized water to the 4 wt % STRX original solution followed by 100 mL seawater. This co-surfactant solution was used as a stock for IFT and phase behavior experiments.

TABLE 4 Co-surfactant alone or Seawater Dilution Nanosurfactant mixture (mL) (mL) Stock 20  0 1:1 10 10 1:2 7 14 1:4 4 16 1:8 2.5 20 1:16 1.5 24

The results are summarized in FIGS. 9A and 9B. As shown in FIG. 9A, the IFT between seawater and crude oil mostly decreases when the concentration of the zwitterionic co-surfactant decreases. As shown in FIG. 9B, the IFT between seawater and crude oil decreases almost monotonically with decreasing concentration of the nanosurfactant mixture, while it reaches a minimum with nanosurfactant mixture at ˜1:4 dilution. The lower the IFT, the larger the capillary number, so the nanosurfactant fluid has an increased ability to mobilize the oil with seawater. The IFT values of the nanosurfactant composition are about two orders of magnitude lower than the IFT values of a fluid with the zwitterionic co-surfactant alone. These results indicate that the efficiency of the nanosurfactant composition to mobilize oil in the reservoir becomes better as it mixes with the pore water in the reservoir.

Further IFT measurements were obtained for the zwitterionic co-surfactant alone and the nanosurfactant composition containing the same amount of zwitterionic co-surfactant with different amounts of petroleum sulfonate. In order to realize the effect of the EOR-2095 concentration on the IFT values, samples with different zwitterionic co-surfactant/EOR-2095 ratios were prepared according to Table 5, where the volume of the zwitterionic co-surfactant solution was held constant in all the samples while varying the amount of EOR-2095.

TABLE 5 Sample # 5 wt % EOR (mL) 4 wt % STRX (mL) SW (mL)  1 0.5 1.3 10  2 0.6  3 0.7  4 0.8  5 0.9  6 1.0  7 1.1  8 1.2  9 1.3 10 1.4 11 1.5 12 1.6 13 1.7 14 1.8

Initially, the samples were prepared for IFT tests by mixing petroleum sulfonate and seawater first, then the zwitterionic co-surfactant was added. The mixtures appeared cloudy, indicating the formation of precipitates. So the measurements were repeated by first mixing the zwitterionic co-surfactant with petroleum sulfonate prepared in fresh water, followed by mixing with seawater. Only half of the samples were prepared for repeating the IFT (FIG. 10). The IFT values of seven nanosurfactant samples with different zwitterionic co-surfactant/petroleum sulfonate ratios were measured as shown in Table 6. The average of the last three readings was used here to present the IFT for each sample. It can be seen that the IFT decreased as the amount of petroleum sulfonate in nanosurfactant was increased as shown in Table 6.

TABLE 6 Sample # EOR/STRX New IFT (mN/m)  1 0.385 0.134  3 0.538 0.104  5 0.692 0.055  7 0.846 0.036  9 1.000 0.022 11 1.154 0.014 13 1.308 0.002

Example 4

To evaluate the role of mineral oil on IFT, a clear brown solution of EOR-2095 (2.5 wt %) was formed by mixing 2 g of oil-free petroleum sulfonate with 19.5 mL of deionized water. Mineral oil was intentionally removed from EOR-2095. The sample was sonicated using a probe sonicator for few minutes followed by using the ultrasonic bath for around 40 minutes with heating the sample at 60° C. This oil-free petroleum sulfonate (2.5 wt %) was used with the zwitterionic co-surfactant (4 wt %) and seawater to prepare two samples with the ratios 1:1:10 and 1:1.25:10 for petroleum sulfonate:the zwitterionic co-surfactant:seawater. The IFT was measured for theses samples for around 30 minutes and compared to the IFT for the zwitterionic co-surfactant-nanosurfactant (stock). Table 7 and FIG. 11 show the role of the mineral oil as well as increasing the ratio of oil-free EOR-2095 to the zwitterionic co-surfactant from 0 to 1.0 (the IFT for the zwitterionic co-surfactant alone is not included, but is in the range of ˜0.3-0.4 mN/m).

TABLE 7 IFT (mN/m) Time Oil-free EOR:STRX oil free EOR:STRX (min) STRX-NS (1:1) (1:1.25)  0 0.060 0.069 0.163  5 0.058 0.089 0.192 10 0.060 0.097 0.218 15 0.066 0.101 0.212 20 0.066 0.108 0.226 25 0.061 0.133 0.214 30 0.072 0.119 0.223

The presence of mineral oil improves the IFT performance of the nanosurfactant by approximately 4-5 times. Mineral oil facilitates the formation of nano-emulsion droplets and containment of the petroleum sulfonates. Also, this suggests that doping the petroleum sulfonates with different types and amounts of oil may play an important role.

Example 5—Testing the Effect of the Soluble Fraction of the Petroleum Sulfonate on the IFT

An experiment was done using two different filtration options to address a concern about how much the soluble fraction of the petroleum sulfonate affects the IFT value with crude oil. About 10 mL of the 5 wt % EOR-2095 was mixed with 12.5 mL deionized water and 100 mL seawater. Sample was set for an hour and precipitated chunks were filtered by a filter paper under vacuum and IFT was measured for this supernatant. The other part of the sample was further filtered using a 0.22 μm syringe filter, then IFT was measured again. Table 8 and FIG. 12 show the IFT results for the supernatant containing EOR-2095 after removal of the gummy precipitate, which was formed in seawater, by filtration through a paper disk. The other part of the supernatant was further filtered by a 0.22 μm syringe filter. The IFT for filtered supernatant was very hard to measure due to the inability to move the oil droplet inside the capillary to the viewing field per the standard procedure. This is an indication of a greater IFT value of the filtered supernatant with crude oil. This confirms that the soluble fraction of petroleum sulfonates in the nanosurfactant structure has no role in the observed IFT reduction of nanosurfactant with crude oil.

TABLE 8 Supernatant Time (min) IFT (mN/m)  0 0.464  5 0.518 10 0.555 15 0.575 20 0.639 25 0.633 30 0.688

IFT for Seawater and the Zwitterionic Co-Surfactant Using Petroleum Sulfonates/Oil

A solution of 5 wt % petroleum sulfonates was prepared by mixing 50 g of EOR-2095 with 950 mL deionized water, as shown in FIG. 13A. The mixture was stirred for about 10 minutes to ensure complete dissolution. About 10 g of petroleum sulfonates/deionized water was added to 50 mL seawater to form precipitates of calcium and magnesium petroleum sulfonate (FIGS. 13B and 13C). The mixture was kept for some time, then the precipitates were filtered and dried under vacuum in the desiccator for several days. After drying, 0.5 g of the waxy sample, as shown in FIG. 13D, was mixed with 5 mL of UTMN crude oil forming petroleum sulfonates/oil, as shown in FIG. 13E. The IFT values of the petroleum sulfonates/oil mixture and seawater only as well as seawater containing the zwitterionic co-surfactant were measured. FIG. 14 shows that the IFT values for the oil-seawater (SW) is greater in the presence of the zwitterionic co-surfactant. This result confirms that the reduction of the oil-seawater IFT is not due to any soluble fraction of petroleum sulfonate that may coexist in the nanosurfactant composition.

Phase behavior experiments were conducted on samples of the zwitterionic co-surfactant alone and the nanosurfactant composition. The experimental results were recorded by visual inspection with the use of UV light (365 nm). About 5 mL from each sample was placed into a pressure tube and sealed tightly to avoid any fluid loss. From each sample, two tubes were prepared; one with the addition of oil and the other without any oil. UTMN crude oil was used and was added gently to the surface of the sample, as shown in FIG. 15. The same procedure was done with deionized water as a control sample. All the tubes were incubated in the oven at 100° C., without any mechanical mixing. At different time intervals (1 h, 2 h, 4 h, 1 d, etc.) the samples were checked and photographs were taken with and without exposure to UV light. FIG. 16A shows the phase behavior results under normal light for the nanosurfactant composition after being incubated in the oven at 100° C. for one week. FIG. 16B shows the phase behavior results under normal light for the fluid containing the zwitterionic co-surfactant alone after being incubated in the oven at 100° C. for one week. FIGS. 16C and 16D show the images of the samples in FIGS. 16A and 16B, respectively, when exposed to UV light.

Comparing the tubes containing the nanosurfactant mixture in contact with crude oil with those containing the co-surfactant alone in contact with crude oil, it can be seen that the samples with the nanosurfactant mixture are much darker, indicating improved formation of oil-in-water emulsions. Comparing the tubes containing the nanosurfactant mixture in contact with crude oil with those containing the co-surfactant alone, but containing no oil indicates that the more pronounced blue light emitted from the nanosurfactant mixture is due to the petroleum sulfonate. When they get in contact with crude oil, samples of the nanosurfactant composition and the fluid with the zwitterionic co-surfactant alone exchange some components with the crude oil into the aqueous phase, with the nanosurfactant composition being more efficient in enhancing that exchange. The above observations further support the ability of the nanosurfactant formulation to enhance the interaction of seawater with crude oil and form oil-in-water emulsions, thus improving recovery.

Further phase behavior experiments were conducted to examine the interaction of the zwitterionic co-surfactant and seawater with the petroleum sulfonates and oil. FIG. 17 shows a schematic of the experimental set up. About five (5) mL the zwitterionic co-surfactant was added in a cylindrical pressure tube with air-tight Teflon lids, followed by the addition of 5 mL of petroleum sulfonates and oil carefully over it. The tubes were incubated in the oven at 100° C. without any mechanical mixing and were checked and photographed with and without being exposed to UV light (365 nm) over different time intervals. The same procedure was followed with seawater, which was prepared as a control sample. FIGS. 18A and 18B show the phase behavior of the zwitterionic co-surfactant and seawater using petroleum sulfonates/oil immediately after set up. After 1 h of incubation at 100° C., the zwitterionic co-surfactant solution was cloudy. FIGS. 18C and 18D show the phase behavior of the zwitterionic co-surfactant and seawater using petroleum sulfonates/oil after one hour at 100° C. These observations signify the ability of the nanosurfactant mixture with zwitterionic co-surfactant to emulsify crude oil without the need of any mechanical mixing, solely by the effect of the reservoir's temperature, due to the extremely low IFT induced by the mixture.

Example 6—Static Sorption Tests

Static sorption tests were conducted to evaluate the nanosurfactant solution with zwitterionic co-surfactant (˜0.2 wt % total active ingredients) and the zwitterionic co-surfactant alone (˜0.1 wt % STRX). Briefly, outcrop Arab-D rock was crushed as fine as possible and sieved to isolate the fraction passing through mesh size 40 and retained mesh size 100. In two 15 mL pressure tubes, about 4.0 g of crushed rock was added with 10 mL of the nanosurfactant mixture. In another two 15 mL pressure tubes, 4.0 g of crushed rock was added with 10 mL of the zwitterionic co-surfactant solution alone. The control samples were separate tubes of 10 mL of the nanosurfactant solution and 10 mL of the zwitterionic co-surfactant solution. Tubes were shaken 10 times and placed in the oven for 24 hours at 100° C. Samples were removed from the oven and 3 mL from the supernatant were collected in centrifuge tubes. Samples were centrifuged for 30 minutes at 3000 rpm, and about 2 mL of the supernatant (uppermost layer) was collected from each centrifuge tube into separate clean test tubes. NMR measurements were done on the supernatant samples. As shown in FIGS. 19A-19C, the ¹H NMR spectra for five different formulations with suppressed/removed H₂O signal are present. The first spectrum from the top is EOR-2095 oil fraction/CDCl₃ for the mineral oil fraction which was removed from EOR-2095 sample and dissolved in deuteriated chloroform. The second spectrum from the top is EOR-2095/10% D₂O for commercial EOR-2095 dissolved in 10% deuterium oxide (D₂O) in water. The third spectrum from the top is STRX/10% D₂O for Surfatex CBS dissolved in 10% deuterium oxide in water. The fourth spectrum from the top is oil-free EOR-2095/10% D₂O for EOR-2095 with originally present mineral oil removed from it and EOR-2095 fractions dissolved in 10% D₂O in water. The last spectrum is NSS20150614/10% D₂O for nanosurfactant formulations in 10 wt % deuterium oxide in water. As shown in FIG. 19A, the nanosurfactant mixture (spectrum at the bottom) has two regions (presented by the left hand side and right hand side dashed rectangles), where the signals of the individual ingredients (peaks at 3-4 ppm coming from Surfatex CBS and 6.6-8.0 ppm from EOR-2095) did not overlap and could be integrated with sufficient accuracy. FIG. 19B and FIG. 19C are expanded visualizations of the ¹H NMR spectra between 6.3 and 8 ppm and between 0 and 4.2 ppm, respectively.

To quantify the loss of surfactants due to adsorption, three 10 mL aliquots of zwitterionic co-surfactant alone (0.2 wt % in synthetic seawater) and three 10 mL aliquots of the nanosurfactant mixture (0.2 wt % of active ingredients) were placed in 15 mL screwcap tubes. Then, powdered Arab-D outcrop rock (4.00 g, 40-100 mesh fraction) was added to the two tubes containing the zwitterionic co-surfactant alone and two tubes containing the nanosurfactant mixture. The tubes were sealed, vigorously shaken and placed in an oven maintained at 100° C. for 24 hours. Following the heating step, the tubes were cooled to room temperature and 3 mL aliquots of the supernatants were withdrawn. The aliquots were placed in disposable polyethylene centrifuge test tubes and centrifuged at 3000 rpm for 30 minutes. Aliquots of the resulting supernatants (0.9 mL) were withdrawn using an Eppendorf micropipette and mixed with a standard solution of maleic acid (0.1 mL, 0.10 g of maleic acid in 11.08 g of D₂O) as an internal reference in scintillation vials. Then about 0.7 mL aliquots of the mixed solutions were transferred to the NMR tubes for measurements. The ¹H-NMR spectra were measured on Bruker Avance spectrometer operating at 400 MHz. To suppress the water peak, each spectrum was induced with an excitation sculpting pulse sequence using the standard (zgespg) program from the Bruker pulse library. A 4000 Hz acquisition window centered at the peak of the water signal (about 4.7 ppm) was used and 64 scans were collected for each sample with a 3 second delay between the scans. The integral of maleic acid (sharp singlet at 6.45 ppm) was given the value of 1000 for every sample and the other integrals were referenced to it. As shown in FIGS. 20A-20B, 21A-21B, 22, and 23, the residual amounts of EOR-2095 and the zwitterionic co-surfactant were determined by dividing the corresponding integral values for the rock exposed samples by the integral values for the control samples (S_(after)/S_(before)), where is Sa corresponds for amount of EOR-2095, Sb for amount of Surfatex CBS, and Sc for combined amount of EOR-2095 with Surfatex CBS.

FIGS. 20A and 20B are ¹H-NMR spectra of samples of the nanosurfactant composition collected before and after contact with powdered Arab-D outcrop rock, respectively. The loss of surfactants due to adsorption when using the nanosurfactant composition (STRX-NS) was quantified from these ¹H-NMR spectra. FIG. 23 is a difference spectrum based on the ¹H-NMR spectra of samples of the nanosurfactant-containing fluid before and after contact with powdered Arab-D outcrop rock, respectively. FIGS. 21A and 21B are ¹H-NMR spectra of samples of a fluid containing the zwitterionic co-surfactant collected before and after contact with powdered Arab-D outcrop rock, respectively. The loss of surfactants due to adsorption when using the zwitterionic co-surfactant (STRX) alone was quantified from these ¹H-NMR spectra. FIG. 22 is a series of difference spectra based on the ¹H-NMR spectra of samples of a fluid containing the zwitterionic co-surfactant collected before and after contact with powdered Arab-D outcrop rock, respectively. The red line is the spectrum for surfactant solution before exposure to rock and the blue line is the spectrum for surfactant solution after exposure to rock. The green line is the difference spectrum that supports the measurement of the amount of surfactant retained on the powdered Arab-D outcrop.

NMR results in FIGS. 21A and 21B showed significant difference between loss values for integrals B and C in the pure the zwitterionic co-surfactant sample, indicating that components of the surfactant containing long aliphatic chain (C₁₁H₂₃ on average, integral C) are adsorbed preferentially. The signals in area B (2.8-4.0 ppm) correspond to the short diamine link derived from N,N-dimethyl-1,3-propanediamine, the aliphatic protons of the hydroxysulfonate head group derived from 3-chloro-2-hydroxypropane sulfonic acid and byproducts derived from the same amine and sulfonic acid. Moreover, as the surfactancy of the zwitterionic co-surfactant results from the presence of long-chain components, using integral C for this surfactant's quantification will provide an estimate better correlated with its activity. Therefore, the present test should be interpreted to show that 23% of the original amount of the zwitterionic co-surfactant (20 mg) was lost to adsorption. This value corresponds to about 1.2 mg of surfactant per 1 gram of rock and is in line with observations reported for other brine-compatible surfactants.

In studies using the nanosurfactant mixture with zwitterionic co-surfactant as shown in FIGS. 20A and 20B, the amounts of aliphatic (area C, 0.30-2.80 ppm) and aromatic (area A, 7.0-8.0 ppm) are significantly reduced after exposure to the rock powder, while signals in area B are not affected as much. As area A (S_(A)) shows only signals resulting from surface active ingredients (long-chain alkylbenzene sulfonates) of EOR-2095, the loss of this surfactant can be reliably quantified by integrating this area alone. Thus, 38% of the original amount of EOR-2095 (10 mg) was lost corresponding to about 1 mg per gram of rock. The amount of zwitterionic co-surfactant that is lost cannot be calculated from direct integration of area C (S_(C)), because of overlapping of signals given by all ingredients, although it can be estimated assuming that the ratio of loss between areas B (S_(B)) and C (S_(C)) remains the same for pure the zwitterionic co-surfactant alone and the nanosurfactant mixture with zwitterionic co-surfactant (STRX-NS). Consequently, the loss of the zwitterionic co-surfactant from the nanosurfactant solution is estimated to be 6%*23%/14%=10% or 1 mg corresponding to 0.25 mg per gram of rock.

In order to assess the effects of mineral oil on the stability and functionality of the nanosurfactant composition (STRX-NS), a set of stability experiments and IFT measurements were conducted following the same procedures and conditions described in the previous sections. FIGS. 24 and 25 and Table 9 summarize the results. As shown in Table 9, when the zwitterionic co-surfactant control was used alone, there was about 16% loss due to adsorption. When the nanosurfactant was used without the zwitterionic co-surfactant but still containing the petroleum sulfonates, then there was a 3% loss due to adsorption. When the nanosurfactant was used without mineral oil, there was a 13% loss due to adsorption as compared to almost no loss when the nanosurfactant was used with mineral oil. FIG. 24 shows the adsorption of the active ingredients onto the rock grains in mg/g and FIG. 25 on a percentage basis for the same samples.

TABLE 9 Before After Loss Loss % % Re- mg mg mg (mg/g) Lost maining Fluid with 20 16.883 3.1169 0.7792 16% 84% Zwitterionic co- surfactant only Nanosurfactant 10 8.6751 1.3249 0.3312 13% 87% without mineral oil Nanosurfactant 10 10     0    0    0 100%  with mineral oil Fluid with 10 9.6543 0.3457 0.0864  3% 97% petroleum sulfonates only

As shown in FIGS. 26A and 26B, removing ˜100% of the mineral oil from the nanosurfactant composition did not affect its stability for the one week period reported. FIG. 26C is a graphical representation of the results from THE IFT evaluation of the nanosurfactant mixture with the petroleum sulfonate, the zwitterionic co-surfactant, and mineral oil. It is also important to note that in FIG. 26C, there is an increase in IFT to 0.43 mN/m caused by removing the mineral oil as compared to the previously reported IFT value of 0.3 mN/m for the zwitterionic co-surfactant-nanosurfactant containing mineral oil data. This result signifies the importance of the mineral oil in aiding the reduction of the IFT induced by the nanosurfactant mixture containing the zwitterionic co-surfactant, and also points out the possibilities of using different oils.

Several sorption tests were also conducted in which total organic carbon was used to quantify the amount of total carbon lost by sorption to the rock. For the nanosurfactant formulation containing the zwitterionic co-surfactant, the tests showed a total carbon loss of 0.88 mg/g after contact with the rock for 14 days at 100° C. This number corresponds to 13% loss of the total mineral oil introduced to the rock that aligns with the NMR results.

Example 7—Oil Recovery Efficiency

The nanosurfactant formulation containing the zwitterionic co-surfactant was subjected to further evaluation of oil recovery using several core plugs (17) from Arab-D reservoir. Routine core analysis was conducted and the petrophysical parameters, such as dimensions, porosity, permeability and pore volume were measured. For the first composite, three core plugs were selected based on their permeability, computed tomography (CT) scan and NMR data. The plugs were arranged as shown in FIG. 27. A schematic illustration of the experimental instrumentation 2800 is shown in FIG. 28. Four different vessels 2801, 2802, 2803, and 2804 are part of the instrumentation 2800. The vessels 2801, 2802, 2803, and 2804 were filled up with dead oil, live oil, synthetic seawater, and nanosurfactant composition, respectively, and the flow of these fluids is controlled by a flow control module 2805. The coreflooding experiment was performed using core sample holder 2806 at reservoir temperature (90° C.) with a flow rate of 0.5 cubic centimeter per minute. During the test, the differential pressure, oil, and water production were recorded in 30 second intervals. This data was used to calculate the incremental oil recovery.

The experimental instrumentation 2800 includes a delta pressure module 2807, capable of delivering low delta pressure, or medium delta pressure, or high delta pressure, and in fluid communication with the core sample holder 2806. The experimental instrumentation 2800 also includes an air inlet 2808 and a water inlet 2809 that are supplied to a confining pressure module 2810. This module 2810 is in fluid communication with the core sample holder 2806. Effluent from the core sample holder 2806 is supplied to a sample analysis module 2811. Not shown in the figure are separators, back-pressure modulators, filters, pressure and temperature sensors, valves, pumps, heating elements and cooling elements that are in fluid communication with various components of the experimental instrumentation 2800 and are known to one of ordinary skill in the art.

About 20 core plugs with known air permeability (k_(air)) >50 milliDarcy)) were selected. These plugs were cleaned and dried and their dry parameters were measured (k_(air), weight, porosity, density). These plugs were saturated with seawater and their wet weight and brine permeability (k_(w)) were measured. Effluent water samples leaving the plugs were subjected to geochemistry analysis (every 4 pore volumes (PV)). Aliquots were provided to an external laboratory for IFT and micromodel experiments and for later imbibition tests. The saturated volumes were calculated and NMR (T2) on seawater-saturated samples were conducted. Core plugs were then saturated with dead UTMN crude oil (4-5 PVs). Samples were centrifuged if needed. The volume of water was measured and the initial water saturation (S_(wi)) was calculated. The plugs were weighed and the volume of oil was calculated. NMR (T2) on oil-saturated plugs was conducted at S_(wi). The effluent oil samples were processed for IFT and micromodel experiments. These plugs were then age-saturated with oil at 90° C. and 1000 psi. Then, the NMR (T2) analysis of the aged samples was conducted. Three sets of core plug composites (3 plugs each) were assembled and dynamic coreflooding was run on two sets of composite core plugs using seawater until reaching residual oil saturation, S_(or). Two core plugs were flooded with nanosurfactant composition, one at S_(wi) and one at S_(or). NMR was conducted on plugs after the dynamic coreflood tests.

FIG. 29A shows the oil recovery results using the nanosurfactant composition. FIG. 29B shows the expanded view of the core flooding results after the original oil-in-place recovery. Around 70% of original oil-in-place was recovered by seawater flooding. After injecting the nanosurfactant formulation, about 7% of additional oil was recovered. The incremental increase in oil recovery took place shortly after injecting the nanosurfactant slug, but at a low rate. Later, when the flow rate was increased to 2 mL/min, there was a significant increase in the rate of oil recovery.

Example 8

A sample nanosurfactant composition, including the petroleum sulfonate, the zwitterionic co-surfactant, and the cationic co-surfactant, and the mineral oil, was formulated by the following process. A PETRONATE® HL/L sodium sulfonate mixture (Sonneborn LLC, Parsippany, N.J.) was used as the source for the petroleum sulfonate and the oil. The petroleum sulfonate mixture had a sulfonate content ranging between 61 wt. % and 63 wt. %. The mixture had an oil content ranging between 31 wt. % and 35 wt. %. A 5 wt. % petroleum sulfonate mixture was prepared by mixing 80 g of the PETRONATE® HL/L sodium sulfonate mixture with 900 mL of deionized water and adjusting the volume to 1,000 mL with additional deionized water. A 4 wt. % aqueous zwitterionic co-surfactant solution was prepared by dissolving 91 g of the STRX co-surfactant (containing cocamidopropyl hydroxysultaine) in 900 mL of deionized water and adjusting the volume to 1,000 mL with additional deionized water once the dissolution was complete. A 4 wt. % aqueous cationic co-surfactant solution was prepared by dissolving 86 g of Cola® Lipid C (containing cocamidopropyl PG-dimonium chloride phosphate, Colonial Chemical, South Pittsburg, Tenn.) in 900 mL of deionized water and adjusting the volume to 1,000 mL with additional deionized water once the dissolution was complete. About 10 mL of the 5 wt. % petroleum sulfonate mixture, 15 mL of the 4 wt. % aqueous zwitterionic co-surfactant solution, 3.2 mL of the 4 wt. % aqueous cationic co-surfactant solution were vigorously mixed to form a suspension. About 530 mL of the synthetic seawater (as shown for example in Table 1, having a TDS concentration ranging between 57,000 ppm and 120,000 ppm) and about 28 mL of the suspension were vigorously mixed to form the sample nanosurfactant composition including the nanoassemblies. The sample nanosurfactant composition had a petroleum sulfonate content of about 0.1 wt. %. The sample nanosurfactant composition had a zwitterionic co-surfactant content of about 0.12 wt. %. The sample nanosurfactant composition had a cationic co-surfactant content of about 0.023 wt. %. The sample nanosurfactant composition had an oil content ranging between about 0.002 wt. % and about 0.02 wt. %. The sample nanosurfactant composition had a nanoassembly content of about 0.223 wt. %.

A control nanosurfactant composition was also formulated using the above process but in the absence of the cationic co-surfactant. The cationic co-surfactant solution was substituted with an identical volume of deionized water. The control nanosurfactant composition had a nanoassembly content of about 0.2 wt. %.

Example 9

The dynamic retention of the nanosurfactant compositions of Example 8 in carbonates was studied by flooding a limestone core plug. The composition of the effluent was determined by total organic carbon (TOC) analysis. The core plug had a 1.5 inch diameter and a 3 inch length. Limestone was used as a carbonate source. The permeability of the core plug was about 200 milliDarcy. The pore volume of the core plug was about 12.5 mL. The core plug was set in a vertical, bottom-to-top configuration. Temperature of the core plug was maintained at about 93° C. Twelve pore volumes of the sample nanosurfactant composition were injected into the core plug at an injection flow rate of about 0.5 mL/min, a core pressure of about 3,200 psi, and a confining pressure of about 4,200 psi. Subsequently, 35 pore volumes of the synthetic seawater were injected into the core plug. The core flooding experiment was repeated using the control nanosurfactant composition.

FIG. 31A shows a graphical representation of a breakthrough curve of a core flooding experiment using the sample nanosurfactant composition of Example 8. The horizontal axis represents the quantity of pore volumes (corresponding to time) injected into the limestone core plug. The left hand side of the vertical axis represents the normalized surfactant concentration determined by conducting TOC analysis of the effluent. The right hand side of the vertical axis represents the total mass of surfactants recovered from the effluent. The sample nanosurfactant composition was injected into the core plug starting at about 1-2 pore volumes represented by the left hand side vertical dashed line. The seawater flush was conducted after about 14 pore volumes of the sample nanosurfactant composition was injected, represented by the right hand side vertical dashed line.

FIG. 31B shows a graphical representation of a breakthrough curve of a core flooding experiment using the control nanosurfactant composition of Example 8. The horizontal axis represents the quantity of pore volumes (corresponding to time) injected into the limestone core plug. The left hand side of the vertical axis represents the normalized surfactant concentration determined by conducting TOC analysis of the effluent. The right hand side of the vertical axis represents the total mass of surfactants recovered from the effluent. The control nanosurfactant composition was injected into the core plug starting at about 0-1 pore volumes represented by the left hand side vertical dashed line. The seawater flush was conducted after about 12 pore volumes of the control nanosurfactant composition was injected, represented by the right hand side vertical dashed line.

FIGS. 31A and 31B show that the sample nanosurfactant composition exhibits less retention on the limestone than the control nanosurfactant composition, resulting in a greater recovery of total mass in the effluent.

FIG. 32 is a graphical representation showing the dynamic retention of the sample nanosurfactant composition and the control nanosurfactant composition of Example 8. The vertical axis represents the dynamic retention on the limestone core plug in units of milligrams of the nanosurfactant per grams of the limestone (mg/g). Without being bound by any theory, it is known that the acceptable range of surfactant retention is known to be less than about 1 mg/g. As shown in FIG. 32, the dynamic retention of the control nanosurfactant composition was about 0.979 mg/g. The dynamic retention of the sample nanosurfactant composition was about 0.451 mg/g, which was >50% less than that of the control nanosurfactant composition. The results show that the cationic co-surfactant in the nanosurfactant composition takes a significant role in reducing the dynamic retention on carbonates.

Ranges may be expressed here as from about one particular value and to about another particular value. Where the range of values is described or referenced here, the interval encompasses each intervening value between the upper limit and the lower limit as well as the upper limit and the lower limit and includes smaller ranges of the interval subject to any specific exclusion provided. Where a method comprising two or more defined steps is recited or referenced here, the defined steps can be carried out in any order or simultaneously except where the context excludes that possibility. While various embodiments have been described in detail for the purpose of illustration, they are not to be construed as limiting, but are intended to cover all the changes and modifications within the spirit and scope thereof. 

What is claimed is:
 1. A method of producing a nanosurfactant composition suitable for injection into a hydrocarbon-bearing formation for enhanced oil recovery operations, the method comprising the steps of: combining a sulfonate surfactant and an oil to form a first mixture; combining the first mixture, a zwitterionic co-surfactant, and fresh water to form a second mixture; combining the second mixture, a cationic co-surfactant, and the fresh water to form a third mixture; and combining the third mixture and saline water to form the nanosurfactant composition, the saline water comprising divalent cations, the nanosurfactant composition comprising a nanoassembly, wherein the nanoassembly has a hydrophobic interior and a hydrophilic exterior, the hydrophobic interior comprising the sulfonate surfactant, a hydrophobic portion of the zwitterionic co-surfactant, a hydrophobic portion of the cationic co-surfactant, and the oil, the hydrophilic exterior comprising a hydrophilic portion of the zwitterionic co-surfactant and a hydrophilic portion of the cationic co-surfactant, wherein the hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant stabilize the nanoassembly by interacting with the divalent cations present in the saline water, wherein the oil facilitates containment of the sulfonate surfactant in the hydrophobic interior of the nanoassembly, wherein the quantity of the sulfonate surfactant in the nanosurfactant composition ranges between 0.1 wt % and 0.25 wt %, wherein the quantity of the oil in the nanosurfactant composition ranges between 0.002 wt % and 0.02 wt %.
 2. The method of claim 1, wherein the nanoassembly has a diameter ranging between 10 nm and 100 nm.
 3. The method of claim 1, wherein the sulfonate surfactant is petroleum sulfonate.
 4. The method of claim 3, wherein the petroleum sulfonate includes one selected from the group consisting of: an alkyl sulfonate, and alkyl aryl sulfonate, and combinations thereof.
 5. The method of claim 1, wherein the zwitterionic co-surfactant is cocamidopropyl hydroxysultaine.
 6. The method of claim 1, wherein the nanosurfactant composition has a zwitterionic co-surfactant content ranging between 0.1 wt. % and 0.2 wt. %.
 7. The method of claim 1, wherein the cationic co-surfactant is cocamidopropyl PG-dimonium chloride phosphate.
 8. The method of claim 1, wherein the nanosurfactant composition has a cationic co-surfactant content ranging between 0.005 wt. % and 0.1 wt. %.
 9. The method of claim 1, wherein the oil is mineral oil.
 10. The method of claim 1, wherein the fresh water has a total dissolved solids concentration less than 3,000 ppm.
 11. The method of claim 1, wherein the saline water has a total dissolved solids concentration greater than 10,000 ppm.
 12. A nanosurfactant composition suitable for injection into a hydrocarbon-bearing formation for enhanced oil recovery operations, the nanosurfactant composition comprising: a nanoassembly, the nanoassembly comprising: a hydrophobic interior, the hydrophobic interior comprising: petroleum sulfonate; a hydrophobic portion of a zwitterionic co-surfactant; a hydrophobic portion of a cationic co-surfactant; and mineral oil; and a hydrophilic exterior, the hydrophilic exterior comprising: a hydrophilic portion of the zwitterionic co-surfactant; and a hydrophilic portion of the cationic co-surfactant; and an aqueous environment, the aqueous environment comprising divalent cations, wherein the hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant are configured to stabilize the nanoassembly by interacting with the divalent cations present in the aqueous environment, wherein the mineral oil is configured to facilitate containment of the petroleum sulfonate in the hydrophobic interior of the nanoassembly, wherein the quantity of the petroleum sulfonate in the nanosurfactant composition ranges between 0.1 wt % and 0.25 wt %, wherein the quantity of the mineral oil in the nanosurfactant composition ranges between 0.002 wt % and 0.02 wt %.
 13. The nanosurfactant composition of claim 12, wherein the nanoassembly has a diameter ranging between 10 nm and 100 nm.
 14. The nanosurfactant composition of claim 12, wherein the zwitterionic co-surfactant is cocamidopropyl hydroxysultaine.
 15. The nanosurfactant composition of claim 12, wherein the cationic co-surfactant is cocamidopropyl PG-dimonium chloride phosphate.
 16. A method for recovering hydrocarbons from a hydrocarbon-bearing formation, the method comprising the steps of: introducing into the hydrocarbon-bearing formation a nanosurfactant composition, the nanosurfactant composition formed by the steps of: combining a sulfonate surfactant and an oil to form a first mixture; combining the first mixture, a zwitterionic co-surfactant, and fresh water to form a second mixture; combining the second mixture, a cationic co-surfactant, and the fresh water to form a third mixture; and combining the third mixture and saline water to form the nanosurfactant composition, the saline water comprising divalent cations, the nanosurfactant composition comprising a nanoassembly, wherein the nanoassembly has a hydrophobic interior and a hydrophilic exterior, the hydrophobic interior comprising the sulfonate surfactant, a hydrophobic portion of the zwitterionic co-surfactant, a hydrophobic portion of the cationic co-surfactant, and the oil, the hydrophilic exterior comprising a hydrophilic portion of the zwitterionic co-surfactant and a hydrophilic portion of the cationic co-surfactant, wherein the hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant stabilize the nanoassembly by interacting with the divalent cations present in the saline water, wherein the oil facilitates containment of the sulfonate surfactant in the hydrophobic interior of the nanoassembly, wherein the quantity of the sulfonate surfactant in the nanosurfactant composition ranges between 0.1 wt % and 0.25 wt %, wherein the quantity of the oil in the nanosurfactant composition ranges between 0.002 wt % and 0.02 wt %; and driving the nanosurfactant composition through the hydrocarbon-bearing formation to displace the hydrocarbons from the hydrocarbon-bearing formation, wherein the nanoassembly reduces interfacial tension between the hydrocarbons and the saline water.
 17. The method of claim 16, wherein the sulfonate surfactant is petroleum sulfonate.
 18. The method of claim 16, wherein the zwitterionic co-surfactant is cocamidopropyl hydroxysultaine.
 19. The method of claim 16, wherein the cationic co-surfactant is cocamidopropyl PG-dimonium chloride phosphate.
 20. The method of claim 16, wherein the oil is mineral oil.
 21. A method of producing a nanosurfactant composition suitable for injection into a hydrocarbon-bearing formation for enhanced oil recovery operations, the method comprising the steps of: combining a sulfonate surfactant, an oil, and fresh water to form a first mixture; combining a zwitterionic co-surfactant and the fresh water to form a second mixture; combining a cationic co-surfactant and the fresh water to form a third mixture; combining the first mixture, the second mixture, and the third mixture to form a fourth mixture; and combining the fourth mixture and saline water to form the nanosurfactant composition, the saline water comprising divalent cations, the nanosurfactant composition comprising a nanoassembly, wherein the nanoassembly has a hydrophobic interior and a hydrophilic exterior, the hydrophobic interior comprising the sulfonate surfactant, a hydrophobic portion of the zwitterionic co-surfactant, a hydrophobic portion of the cationic co-surfactant, and the oil, the hydrophilic exterior comprising a hydrophilic portion of the zwitterionic co-surfactant and a hydrophilic portion of the cationic co-surfactant, wherein the hydrophilic portion of the zwitterionic co-surfactant and the hydrophilic portion of the cationic co-surfactant stabilize the nanoassembly by interacting with the divalent cations present in the saline water, wherein the oil facilitates containment of the sulfonate surfactant in the hydrophobic interior of the nanoassembly, wherein the quantity of the sulfonate surfactant in the nanosurfactant composition ranges between 0.1 wt % and 0.25 wt %, wherein the quantity of the oil in the nanosurfactant composition ranges between 0.002 wt % and 0.02 wt %. 